In loading operation of the loader such as the tractor shovel and the like, the earth and sand are loaded into a dump trucks, hoppers and the like. Namely, the earth and sand shoveled by a bucket of such loader are discharged into the dump truck, hopper and the like. In case that the loader is a tractr shovel, after completion of discharging of the earth and sand, an operator of the tractor shovel backs the tractor shovel. At this time, in order to efficiently perform shoveling of the earth and sand in a minimum of time in the next cycle, the operator moves a boom of the tractor shovel downward, while he so changes the direction of the bucket that the bucket having been tilted downward for discharging the earth and sand is tilted back to be kept horizontal so as to facilitate the shoveling operation of the bucket conducted along the ground in the next cycle.
Namely, after completion of discharging of the earth and sand, the operator must confirm a rearward safety of the tractor shovel when he backs the tractor shovel. In addition to such confirmation of the rearward safety of the tractor shovel, the operator must confirm a frontward safety of the tractor shovel in an operation in which the bucket is tilted back to be kept horizontal at a time when the bucket is brought into contact with the ground. Consequently, at this time, the operator of the tractor shovel must perform simultaneously a plurality of actions such as: a steering action of the tractor shovel; controlling actions of levers for lowering a boom; and controlling actions for stopping a tilting operation of the bucket so as to keep the bucket horizontal. Therefore, the operator of the tractor shovel must be a skilled one. In addition, in the above tilting operation of the bucket, since the bucket is tilted back to be kept horizontal through visual observation of the operator, the horizontal position of the bucket varies in each cycle to make an operational accuracy of the shoveling of the bucket poor. This is disadvantageous to the tractor shovel.
As is already well known, in order to eliminate such disadvantage, a bucket leveler device has been employed in the tractor shovel.
Such conventional bucket leveler device is constructed of: a signal issuring portion "A" shown in FIGS. 1 and 2 of the accompanying drawings; and a bucket control lever return mechanism portion "B" shown in FIG. 1.
As shown in FIGS. 1 and 2, in the signal issuing portion "A" of the bucket leveler device: a limit switch 33 is mounted on a side portion of a cylinder 31 of a bucket cylinder unit 27 for controlling the position of a bucket 23 with respect to a boom 22; and a position detecting element 34 is so mounted on a piston rod 29 of the bucket cylinder unit 27 as to turn on the limit switch 33 at a time when the bucket cylinder unit 27 is extended by a predetermined length, whereby the limit switch 33 having been turned on issues a signal.
On the other hand, as shown in FIGS. 1 and 3, in the bucket control lever return mechanism portion "B" of the bucket leveler device a guide plate 44 is mounted on a bucket control lever 43. The bucket control lever return mechanism portion "B" is also provided with a lever detent mechanism "C" in which: a roller 50 is pressed against the guide plate 44 under the influence of a resilient force exerted by a tension spring 49; the guide plate 44 in its contour and the tension spring 49 in its stiffness are so determined as to make it possible to hold the bucket control lever 43 at its full-stroke position after the bucket control lever 43 is moved to such full-stroke position thereof. In the bucket control lever return mechanism portion "B" of the bucket leveler device, there is further provided a leveler solenoid 46 for electrically unlocking the lever detent mechanism "C".
In the signal issuing portion "A" of the bucket leveler device, when the bucket cylinder unit 27 is extended by the predetermined length, the limit switch 33 is turned on to issue an electrical signal for energizing the leveler solenoid 46. The thus energized leveler solenoid 46 unlocks the lever detent mechanism "C" to permit a bucket control valve 35 to be moved to its neutral position from a tilt position thereof under the influence of a resilient force exerted by a return spring incorporated in the bucket control valve 35.
Namely, once the operator has moved the bucket control lever 43 to its full stroke position at a time when the operator backs the tractor shovel, it is possible for the operator to permit the bucket 23 to continue its tilting action even if he frees his hand from the bucket control lever 43 thereafter. When the bucket cylinder unit 27 has been extended by the predetermined length for directing the bucket 23 in a horizontal direction in general, the bucket control lever 43 automatically returns to its neutral position to stop the tilting action of the bucket 23.
Consequently, the operator can concentrate his attention on the lowering operation of the boom 22 and the steering operation of the tractor shovel through his hands and on the rearward safety through this eyes. As is clear from the above description, the bucket leveler device is indispensable to the loader such as the tractor shovel and the like.
In a brief summary, the operations of the bucket leveler device in the prior art are as follows: turn on the limit switch 33.fwdarw. actuate the leverler solenoid 46.fwdarw. oerate the bucket control lever 43.fwdarw. actuate the bucket control valve 35.fwdarw. control the oil pressure in the bucket cylinder unit 27.fwdarw. and stop the bucket 23. In the above operations of the bucket leveler device, there are slight time lags between the operations. However, these slight time lags amount to a considerable time lag of the order of naught point several seconds in the entire bucket leveler device.
On the other hand, since the delivery of an oil pump in a hydraulic circuit employed in the bucket leveler device depends on the engine speed, an angular velocity of the bucket 23 in its tilting-back action varies as the engine speed varies.
Consequently, the stop position of the bucket 23 varies according to variation of the engine speed, which produces the time lags and variation of the angular velocity of the bucket 23 in its tilting-back action.
Namely, when the engine speed is relatively low, the bucket 23 is held at a high position slightly higher than a desired position. On the other hand, when the engine speed is relatively high, the bucket 23 is held at a position higher than the above high position. As described above, the stop position of the bucket 23 varies when the engine speed varies.
Since the stop position of the bucket 23 varies as described above, shoveling of thin surface layers of products and the soil by the use of the bucket 23 adversely affect the products and the soil. In addition, due to such variation of the stop position of the bucket 23, the cutting edge of the bucket 23 is unevenly worn to cause economical disadvantages. In order to prevent the stop position of the bucket 23 from varying, hitherto, the operator keeps the engine speed constant, or controls the stop position of the bucket 23 in a very sensitive manner, which causes the operator to be very tired. Such operator's fatigue is another problem inherent in the conventional bucket leveler device.